Electrode catheters have been in common use in medical practice for many years. They are used to stimulate and map electrical activity in the heart and to ablate sites of aberrant electrical activity.
In use, the electrode catheter is inserted into a major vein or artery, e.g., femoral. artery, and then guided into the chamber of the heart which is of concern. Within the heart, the ability to control the exact position and orientation of the catheter tip is critical and largely determines how useful the catheter is.
In certain applications, it is desirable to have an irrigated tip catheter in order to cool the tip electrode at the site of ablation and to prevent thrombus.
A typical ablation procedure involves the insertion of a catheter having a tip electrode at its distal end into a heart chamber. A reference electrode is provided, generally taped to the skin of the patient. RF (radio frequency) current is applied to the tip electrode, and current flows through the media that surrounds it, i.e., blood and tissue, toward the reference electrode. The distribution of current depends on the amount of electrode surface in contact with the tissue as compared to blood, which has a higher conductivity than the tissue. Heating of the tissue occurs due to its electrical resistance. The tissue is heated sufficiently to cause a lesion. Heating of the electrode results from conduction from the heated tissue. While the blood circulating around the ablation electrode tends to cool it, a stagnant area between the electrode and the tissue may be heated to such a temperature that a thin coating of blood protein forms on the surface of the tip electrode. This can cause an impedance rise and/or a thrombus that could become an embolus. When this occurs, the catheter should be removed and the tip electrode cleaned.
When RF current is applied to an ablation electrode in good contact with the endocardium to create a lesion, the amount of power delivered is limited by the heating of the electrode in order to prevent char and thrombus. The resulting lesion tends to be hemispherical, usually about 6 mm in diameter and about 3 to 4 mm deep.
When a tip electrode is irrigated, e.g., with room temperature saline, the tip electrode is cooled by the flow of saline through it and the surface of the electrode is flushed. Because the strength of the RF current is no longer limited by the interface temperature, current can be increased. This results in lesions which tend to be larger and more spherical, usually measuring about 10 to 12 mm.
Current irrigated catheters utilize either closed or open fluid systems. Open irrigation fluid systems use holes placed in specific locations around the tip electrode to distribute the irrigation fluid. These designs do not provide uniform fluid distribution along the outer surface of the tip electrodes. Additionally, the irrigation fluid is projected far from the tip electrode and does not provide a uniform and complete boundary layer from the surrounding blood. Accordingly, it is desirable to provide an irrigated catheter with a generally complete and uniform boundary layer reducing direct blood contact with the tip electrode during the application of RF energy. With maintained irrigation fluid-to-tip electrode contact, such an improved catheter will provide increased heat loss including convective heat loss resulting in more efficient cooling and thrombus prevention.